IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 65, NO. 2, FEBRUARY 2017 489 Pattern and Gain Characterization Using Nonintrusive Very-Near-Field Electro-Optical Measurements Over Arbitrary Closed Surfaces Kamal Sarabandi, Fellow, IEEE, Jihun Choi, Student Member, IEEE, Ali Sabet, and Kazem Sabet, Senior Member, IEEE

Abstract— A nonintrusive near-field measurement technique of course the direct far-field measurement that can be accom- for 3-D radiation pattern and gain characterization of antennas is plished in outdoor ranges or in anechoic chambers [1]Ð[3]. The presented. The method is of particular interest for low-frequency use of such techniques with low-frequency antennas operating antennas for which anechoic chambers cannot be developed and far-field measurements are rather cumbersome. Nonintrusive, at high frequency (HF) or low very HF (VHF) bands is limited broadband measurements are performed using an extremely by a number of factors. At these frequencies, the sizes of small all-dielectric electro-optical probe to measure the tangential anechoic chambers and the absorbers become prohibitively electric fields of an under test (AUT) at a very-near large. Outdoor ranges require a vast space and tall towers, and surface enclosing the antenna. Far-field radiation is computed the effect of ground must be carefully examined for accurate from a new near-field to far-field transformation formulation using only the tangential components of the electric field over an measurements. Special approaches to measure HF and VHF arbitrary surface. This procedure employs reciprocity theorem antennas have been reported in [4]Ð[9]. These involve scale and the excited electric current on the surface of a perfect modeling, utilization of aircraft-towed transmitters in antenna electric conductor enclosure having the same geometry as the measurements, design of ground-reflection ranges to minimize scanned surface and illuminated by a plane wave. In this way, the contribution of reflection from the ground, and near-field a full spherical radiation pattern and gain of the AUT are easily computed without expensive computation and truncation scanning. All of these require substantial time and effort to errors. To demonstrate the proposed approach, a miniaturized perform the measurements as well as a specially designed low very high frequency antenna operating at 40 MHz with very-large, expensive anechoic chamber in the case of indoor dimensions 0.013λ0×0.013λ0 × 0.02λ0 is utilized. The far-field measurements. results from our approach are shown to be in good agreement In order to circumvent these difficulties, a nonintrusive near- with those obtained from full-wave simulation and direct far-field measurement performed in an elevated outdoor range. field measurement system [10] using an all-dielectric and very small electro-optical (EO) probe is presented in this paper. The Index Terms— Antenna measurements, antenna radiation advantage of the EO probe is that it can be placed very close patterns, electrically small antennas, electro-optical (EO) system, high frequency (HF)/very HF (VHF) antennas, near-field to the antenna surface where the near field is very strong. For measurements. a nonmetallic room that is sufficiently larger than the antenna and for the antenna sufficiently away from obstacles in the I. INTRODUCTION room, the use of absorbers is not critical. The criteria for CCURATE measurement of fundamental antenna para- setting object distances from the antenna are established by Ameters, such as input impedance, bandwidth, radiation monitoring the reactive part of the antenna input impedance as pattern, and gain is very important for all applications. antenna height above ground is changed and nearby obstacles Knowledge of the actual antenna performance plays an impor- are pushed away from the antenna surrounding. Once a stable tant role in designing real-world communication or condition